The Global Positing System (GPS) is a satellite-based navigation system that continuously transmits timing, frequency, and satellite position information to potential users. The GPS consists of a full constellation of twenty-four (24) satellites in half geosynchronous orbits. The position of the GPS satellites is controlled and monitored by the Department of Defense (DoD). GPS satellites continuously emit coded GPS signals. The GPS signal contains timing information that allows a user to determine the time elapsed for the GPS signal to transverse the distance between the GPS satellite and the user (the platform). By knowing the time the GPS signal left the GPS satellite, the time the GPS signal arrived at the user, and the speed of the GPS signal, the user can determine the distance from itself to the GPS satellite. By knowing the position of the GPS satellite (ephemeris data), and the distance from itself to the GPS satellite, the user can successfully triangulate its own position.
The GPS signal emitted by the satellites contains an L-band carrier component (L1) transmitted at a frequency of 1575.42 MHz. The L1 carrier component is modulated by a coarse acquisition (C/A) pseudorandom (PRN) code component and a data component. The PRN code provides timing information for determining when the GPS signal was broadcast. The data component provides information such as the satellite's orbital position. The carrier component allows a receiver to more easily acquire the GPS signal.
Position determination using a conventional GPS receiver is well known in the art. In conventional GPS, a receiver makes ranging measurements between an antenna coupled to the receiver and each of at least four GPS satellites in view. The receiver makes these measurements from the timing information and the satellite orbital position information obtained from the PRN code and data components of each GPS signal received. By receiving at least four different GPS signals, the receiver can make fairly accurate time and position determinations.
However, a conventional GPS receiver only allows a user to determine actual location to within tens of meters. This accuracy is not suitable for applications which require extreme precision, such as attitude determination for moving vehicles.
A more accurate version of a GPS receiver is an Ordinary Differential GPS receiver. Position determination using Ordinary Differential GPS receiver is also well known in the art. It involves the same kind of ranging measurements that are made with a conventional GPS receiver, except that a ground reference receiver at a precisely known location is utilized. Ideally, satellite ranging errors will affect the position determinations made by the user's receiver in the same way as they will the position determinations made by the nearby ground receiver. Since the location of the ground receiver is already known, the ground receiver can compare the position determination it has calculated with the actual known position. As a result, the ground receiver can accurately detect ranging errors.
From these errors, the ground receiver can compute suitable corrections which are transmitted by data link to the user's receiver. The user's receiver can then apply the corrections to its own ranging measurements so as to provide accurate real time position determinations.
However, even with the Ordinary Differential GPS receiver, the position determinations are only accurate to within several meters. Since, as indicated earlier, attitude determination must be extremely accurate, extending Ordinary Differential GPS to attitude determination is not feasible.
An even more accurate form of a GPS receiver is a Carrier Phase Differential GPS receiver. This form of the GPS receiver utilizes the 1575.42 MHz (L1) carrier component of the GPS signal on which the PRN code and the data component are superimposed. Carrier Phase Differential GPS involves generating position determinations based on the measured phase differences at two different antennas for the carrier component of a GPS signal. However, this technique initially requires determining how many integer wavelengths of the carrier component exist between the two antennas at a particular point in time. This is called integer ambiguity resolution.
As described, a Carrier Phase Differential GPS receiver must be able to accurately detect the carrier signal to make precise determinations of phase differences and numbers of wavelengths. Under weak signal conditions, the carrier cannot be properly detected (a state known as the GPS State 3). The conventional solution for this problem has been the use of Kalman filtering. Kalman filtering is not one unique method, but is a generic name for a class of state estimators based on noisy measurements. Kalman filtering can be implemented as a specific algorithm on a general-purpose mainframe/ mini/ microcomputer operating in a batch mode or it can be implemented on a dedicated system using either DSP, ASIC, or custom VLSI processors in a real-time operating mode.
In GPS receivers, Kalman filters estimate systematic errors from the GPS navigation data. Kalman filters are able to provide the GPS with 1 Hz error updates. Quicker updates would provide more accurate estimations of carrier signals and, thus, more accurate phase determinations used, for example, to calculate attitude of the platform. Nevertheless, even when the GPS carrier signal is strong enough to be detected (a state known as the GPS State 5), conventional GPS receiver designs have the problem of carrier cycle slip in their carrier measurements. As such, GPS carrier estimation is useful even when the carrier signal is not jammed or indiscernible.
Thus, there is a need for an improved system and method to estimate the waveform of the carrier signal of global positioning systems (GPS). Further, there is a need for more accurate location and attitude determinations when tracking of the carrier signal is not possible (GPS State 3). Further still, there is a need to have an external measurement to detect any carrier cycle slips when detection of the carrier signal is possible (GPS State 5).